U.S. Vice President Joe Biden says the mass shooting in Orlando, Florida is "becoming clearer and more straight-forward" as investigators try to determine the motive behind the attack that killed 49 people and wounded 53 others.

Biden did not elaborate Tuesday, but said that in the coming days President Barack Obama would say more. Obama is due to visit Orlando on Thursday.

The president in his own comments Tuesday said the shooting was carried out by an "angry, disturbed, unstable young man who became radicalized." He said there was no indication that any foreign terrorist group directed 29-year-old Omar Mateen in his attack on a gay nightclub.

Obama said U.S. law enforcement officials "are doing everything to stop these kinds of attacks," but are "sobered" by the difficulty in trying to detect "these lone actors" beforehand.

U.S. Homeland Security Secretary Jeh Johnson said authorities are continuing to collect information about Mateen's life, and called for "meaningful, responsible" gun control laws.

"It's something that I think the American public and the Congress has to face and has to address," Johnson said on CBS This Morning.

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In Photos: Worldwide Reaction to Pulse Orlando Massacre

Patrons at the Pulse nightclub said they had frequently seen Mateen there over the past several years. Others said they encountered Mateen, twice married, once divorced, and with a three-year-old son, on the gay chat and dating app "Jack'd."

One man, Ty Smith, said he remembered seeing Mateen at Pulse at least a dozen times.

"Sometimes he would go over in the corner and sit and drink by himself, and other times he would get so drunk he was loud and belligerent," Smith said.

Another man, Jim Van Horn, 71, said Mateen was a "regular" at Pulse. "He was trying to pick up people. Men," Van Horn told The Associated Press. Van Horn and others familiar with the gay club scene in Orlando say that Mateen may have been conflicted about his sexuality and acted out of anti-gay hatred in carrying out the rampage.

Mateen's father, Seddique Mateen, has told reporters his son became visibly upset after seeing two men kissing on a recent visit to Miami, another Florida city in the southeastern U.S.

In this photo made from video, Seddique Mateen speaks in his Florida home to reporters about his son, Omar Mateen, the gunman in the attack on an Orlando, Fla., gay night spot. “I did not know that he had a grudge in his heart,” Mateen said of his son.

Law enforcement authorities say that twice in recent years they investigated Mateen's connections with terrorist groups, but concluded no charges were warranted and dropped further inquiries.

Now, the Federal Bureau of Investigation, the country's top law enforcement agency, is taking a look back to see if signs of Mateen's radicalization were missed, even as it collects volumes of information about his recent activities leading up to his assault on the club.

Mateen's shooting spree bore an ominous connection to other tragic mass killings in the United States carried by out by attackers using legally available AR-15 semi-automatic rifles.

Mateen used the weapon to shoot a huge number of rounds at late-night revelers at the club, many of them of attracted by Latin-themed dance music last Saturday and into the early hours of Sunday when the attack occurred.

Mateen legally bought the powerful assault weapon "in the past few days," according to authorities. The AR-15 allows shooters to fire many rounds as fast as they can pull the trigger. The AR-15 was prohibited for sale in the United States in 1994, but the ban was lifted in 2004 and has not been renewed.